1. Field of the Invention
The present invention relates to a power transformer/inductor.
In all transmission and distribution of electric energy, transformers are used for enabling exchange between two or more electric systems normally having different voltage levels. Transformers are available for powers from the VA region to the 1000 MVA region. The voltage range has a spectrum of up to the highest transmission voltages used today. Electromagnetic induction is used for energy transmission between electric systems.
Inductors are also an essential component in the transmission of electric energy in for example phase compensation and filtering.
The transformer/inductor related to the present invention belongs to the so-called power transformers/inductors having rated outputs from several hundred kVA to in excess of 1000 MVA and rated voltages of from 3–4 kV to very high transmission voltages.
2. Discussion of the Background
Generally speaking the main object of a power transformer is to enable the exchange of electric energy, between two or more electric systems of mostly differing voltages with the same frequency. Conventional power transformers/inductors are e.g. described in the book “Elektriska Maskiner” by Fredrik Gustavson, page 3–6-3–12, published by The Royal Institute of Technology, Sweden, 1996.
A conventional power transformer/inductor includes a transformer core, referred to below as a core, formed of laminated commonly oriented sheet, normally of silicon iron. The core is composed of a number of core legs connected by yokes. A number of windings are provided around the core legs normally referred to as primary, secondary and regulating winding. In power transformers these windings are practically always arranged in concentric configuration and distributed along the length of the core leg.
Other types of core structures occasionally occur in e.g. so-called shell transformers or in ring-core transformers. Examples related to core constructions are discussed in DE 40414. The core may be made of conventional magnetizable materials such as said oriented sheet and other magnetizable materials such as ferrites, amorphous material, wire strands or metal tape. The magnetizable core is, as known, not necessary in inductors.
The above-mentioned windings constitute one or several coils connected in series, the coils of which having a number of turns connected in series. The turns of a single coil normally make up a geometric, continuous unit which is physically separated from the remaining coils.
A conductor is known through U.S. Pat. No. 5,036,165, in which the insulation is provided with an inner and an outer layer of semiconducting pyrolized glassfiber. It is also known to provide conductors in a dynamo-electric machine with such an insulation, as described in U.S. Pat. No. 5,066,881 for instance, where a semiconducting pyrolized glassfiber layer is in contact with the two parallel rods forming the conductor, and the insulation in the stator slots is surrounded by an outer layer of semiconducting pyrolized glassfiber. The pyrolized glassfiber material is described as suitable since it retains its resistivity even after the impregnation treatment.
The insulation system, partly on the inside of a coil winding and partly between coils/windings and remaining metal parts, is normally in the form of a solid- or varnish based insulation and the insulation system on the outside is in the form of a solid cellulose insulation, fluid insulation, and possibly also an insulation in the form of gas. Windings with insulation and possible bulky parts represent in this way large volumes that will be subjected to high electric field strengths occurring in and around the active electric magnetic parts belonging to transformers. A detailed knowledge of the properties of insulation material is required in order to predetermine the dielectric field strengths which arise and to attain a dimensioning such that there is a minimal risk of electrical discharge. It is important to achieve a surrounding environment which does not change or reduce the insulation properties.
Today's predominant outer insulation system for conventional high voltage power transformers/inductors include cellulose material as the solid insulation and transformer oil as the fluid insulation. Transformer oil is based on so-called mineral oil.
Conventional insulation systems are e.g. described in the book “Elektriska Maskiner” by Fredrik Gustavson, page 3–9-3–11, published by The Royal Institute of Technology, Sweden, 1996.
Additionally, a conventional insulation system is relatively complicated to construct and special measures need to be taken during manufacture in order to utilize good insulation properties of the insulation system. The system must have a low moisture content and the solid phase in the insulation system needs to be well impregnated with the surrounding oil so that there is minimal risk of gas pockets. During manufacture a special drying process is carried out on the complete core with windings before it is lowered into the tank. After lowering the core and sealing the tank, the tank is emptied of all air by a special vacuum treatment before being filled with oil. This process is relatively time-consuming seen from the entire manufacturing process in addition to the extensive utilization of resources in the workshop.
The tank surrounding the transformer must be constructed in such a way that it is able to withstand full vacuum since the process requires that all the gas be pumped out to almost absolute vacuum which involves extra material consumption and manufacturing time.
Furthermore the installation requires vacuum treatment to be repeated each time the transformer is opened for inspection.